A device of this type is described in an article entitled "Mesure a distance de la distribution de temperature a l'aide d'une fibre optique" (Remotely measuring temperature distribution using an optical fiber) by Hartog et al taken from "European Conference on Optical Communications" (E.C.O.C.), Cannes 1983, session A IV--Propietes des fibres (III) (Fiber properties III), pages 215 to 220. With this device, proposals are made to measure the temperature at measurement points which are distributed along a liquid-core optical fiber. Excitation pulses are provided by a laser and the returned optical radiation is constituted by optical signals backscattered from the measurement points. The returned optical radiation is processed by measuring the level of said signals which level is a function of the temperature at the measurement points. The time taken by the signals to reach the station depends directly on the speed of propagation of optical radiation in the fiber: the time taken is therefore representative of the positions of the measurement points along the fiber. A curve can then be plotted as a function of time showing variations in the level of the backscattered optical signals as received by the station. After taking account of diffusion in the fiber, the temperature of the measurement points can readily be deduced as a function of their positions along the fiber.
In this apparatus, the emission of backscattered optical signals coincides in time with the instants at which the measurement points are excited. However, it is obvious that the method described in the article cannot be used when the characteristic of the backscattered optical radiation which is to be measured has a duration which is long relative to the propagation time of optical radiation in the fiber. By way of example, an optical fiber apparatus of this type in which the characteristic to be measured in the return radiation is the lifetime of fluorescence in a rare earth dopant of the fiber core cannot be made in practice since the lifetime in question is about 500 microseconds, which corresponds to a path length of 100 km along the optical fiber. In this case, the spectroscopical information returning from various measurement points would be superposed in time on reception by the processing circuit, and such information would therefore be unusable since it would be impossible to localize any of the measurement points.